Omnidirectional slot antenna

Abstract: There is provided a small-sized and simplified horizontally polarized antenna apparatus which forms an omnidirectional pattern in the horizontal plane. The radiation field in the horizontal plane becomes continuous and a horizontally polarized omnidirectional radiation pattern can be obtained in the horizontal plane by forming radiation slots at opposing positions on a grounded hollow body and exciting the slots out of phase.

Description: The present invention relates to a horizontally polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane. Figs. 1(a) and 1(b) schematically illustrate a configuration of a horizontal polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane explained in Chapter 12 of "VHF Antenna" written by Uchida and Mushiake, and issued by the Production Technology Center (March, 1977). Fig. 1(a) is a perspective view and Fig. 1(b) is a top plan view with electric field distribution indicated by arrows. In these figures, the numeral 50 designates a dipole antenna and the symbol I indicates a current flowing through the dipole. Next, operations will be explained. A grounded conductor 51 includes four surfaces and a dipole antenna 50 is arranged at each surface. The dipole antenna 50 is arranged in parallel to the horizontal surface to excite a horizontally polarized wave. A plurality of dipole antennas may be arranged in the vertical direction. Amplitudes of currents flowing through the dipole antennas in the same height are equal, but phases thereof are sequentially different by 90 degrees. A dipole antenna 50 has a figure of-8 type radiation directivity, but substantially horizontally polarized omnidirectivity can be obtained through a combination of the four dipole elements. Figs. 2(a) - 2(c) show a conventional slot antenna indicated in "X-band omnidirectional double-slot array antenna" by T. Takeshima, ELECTRONIC ENGINEERING, No. 39, pp. 617-621 (October, 1967). These figures schematically illustrate a configuration of a horizontally polarized antenna apparatus which has an omnidirectional pattern in the horizontal plane (rectangular waveguide slot antenna). Fig. 2(a) is a perspective view, Fig. 2(b) is a sectional view along the line A-A and Fig. 2(c) is a side elevation. In Figs. 2(a) - 2(c), numeral 60 designates a radiation slot; 61 a waveguide; and 62 a flange. The principle in operation of the rectangular waveguide slot antenna shown in Figs. 2(a) - 2(c) will be explained with reference to Figs. 3(a) and 3(b). Fig. 3(a) is a diagram illustrating a distribution of magnetic field inside the waveguide 61. Fig. 3(b) is a cross-sectional view along the line A-A illustrating a distribution of magnetic field inside the waveguide and a current flowing along the side surface. Such distributions of magnetic field and current as illustrated in Figs. 3(a) and 3(b) can be realized by short-circuiting the end portions of the waveguide. Electromagnetic waves propagated along the rectangular waveguide 61 excite the radiation slots 60 to radiate electromagnetic waves if the radiation slots 60 are provided in parallel with the waveguide axis at the positions offset from the center of the H plane of the rectangular waveguide 61. In this case, the radiation slots 60 are excited by providing each of the radiation slots 60 at a position where the magnetic field inside the waveguide 61 becomes maximum. An amount of electromagnetic wave radiation can be adjusted by changing the position of each radiation slot 60. In order that the waveguide slot antenna shown in Figs. 2(a) - 2(c) may be used as a horizontally polarized omnidirectional antenna, the radiation slots 60 are provided, as shown in Fig. 4(a), on the front and rear H planes of the waveguide 61. Then, a distribution of electric field in the horizontal plane changes as shown in Fig. 4(b). The radiation slots 60 are excited out of phase and the radiation field becomes continuous in the horizontal plane. As a result, a theoretically omnidirectional directivity can be realized. However, if, as shown in Fig. 2(a), two radiation slots are formed symmetrically on the front and rear surfaces, two radiation slots can be excited in the same phase by arranging the radiation slots in symmetrical positions of the waveguide 61 with respect to the center thereof at an interval of λg/2 (λg is a wavelength in the waveguide). Therefore, a vertically symmetrical pattern can be obtained in the direction of  = ±90° (in Fig. 4(a)), while a beam tilt is generated in the direction of  = 90° + α and  = 0° and 180° in Fig. 4(a) due to an array factor of the radiation field of the two radiation slots. Accordingly, on the x-y plane, a gain difference is generated in the direction of  = ±90°, 0° and 180° and a ripple in the horizontal plane becomes significant whereby no omnidirectivity can be achieved. In the case where one radiation slot is provided in a position offset from the center of the H plane of the waveguide, no symmetrical configuration is formed and actually no omnidirectivity can be obtained The present invention has been proposed to overcome the problems described above and it is therefore an object of the present invention to provide a small-sized horizontally polarised omnidirectional antenna having a simplified configuration. US-A-4,247,858 discloses an antenna in accordance with the pre-characterising portion of claim 1 and which comprises a hollow body provided with radiation slots and filled with dielectric material. According to the present invention there is provided an antenna apparatus having radiation slots arranged at opposite positions on a grounded conductive rectangular hollow body formed of conductive plates, said hollow body being provided with a dielectric material therein, said radiation slots being excited out of phase to form an omnidirectional radiation pattern in a plane perpendicular to the longitudinal axis of said hollow body, characterised in that a through-hole is formed extending between said radiation slots and in that semi-cylindrical conductor plates are respectively mounted to the conductive plates parallel to the longitudinal axis of said hollow body for the purpose of reducing any influence of waves diffracted at the edges of the conductive plates. At least one conductive bar can be provided around the radiation slots to connect the opposing conductive plates, whereby any unwanted waveguide mode can be suppressed. It is possible to provide horn-type conductive plates on the conductive plates perpendicular to the longitudinal axis of the hollow body. The horn-type conductive plates enable a beam width in a plane including the longitudinal axis'to be reduced without changing the size and position of the radiation slots and an omni-directional high-gain radiation pattern to be achieved in the plane perpendicular to the longitudinal axis. In the above aspects, since the slots are excited out of phase, the electrical field radiated from the radiation slots becomes continuous in a plane perpendicular to the hollow body, for instance, in the horizontal plane and therefore an omnidirectional radiation pattern can be obtained in the horizontal plane. The filling of the hollow body with a dielectric material means that the antenna apparatus can be manufactured in a small size due to a wavelength shortening effect of the dielectric material. The presence of the through-hole in the dielectric material between the radiation slots means that the radiation slots become longer to thereby resonate at the same frequency, a beam width becomes narrow in the plane perpendicular to the hollow body and a gain can be increased. The present invention will be further described by way of example with reference to the accompanying drawings, in which:- Fig. 1(a) is a perspective view of a conventional omnidirectional antenna apparatus. Fig. 1(b) is a plan view of the antenna apparatus of Fig. 1(a), illustrating a distribution of electric field. Fig. 2(a) is a perspective view illustrating another conventional omnidirectional antenna apparatus. Fig. 2(b) is a cross-sectional view taken along the line A-A of Fig. 2(a). Fig. 2(c) is a side elevation of the antenna apparatus of Fig. 2(a). Fig. 3(a) illustrates a distribution of magnetic field in the antenna apparatus of Fig.2 (a). Fig 3(b) illustrates directions of current and magnetic field at the cross-sectional taken along the line A-A of Fig. 3(a). Fig. 4(a) is a diagram for explaining directivity of the antenna apparatus of Fig. 2(a). Fig. 4(b) illustrates a horizontal distribution of electric field established by the antenna apparatus of Fig. 4(a). Fig. 5(a) is a perspective view of an antenna apparatus. Fig. 5(b) is a cross-sectional view taken along the line A-A of Fig. 5(a). Fig. 5(c) is a cross-sectional view taken along the line B-B of Fig. 5(a). Fig. 6 is a diagram for explaining operations of the antenna apparatus of Fig. 5(a). Fig. 7 is a graph illustrating a gain in the azimuth direction of the antenna apparatus of Fig. 5(a). Fig. 8(a) is a perspective view of an embodiment of an antenna apparatus of the present invention. Fig. 8(b) is a cross-sectional view taken along the line A-A of Fig. 8(a). Fig. 8(c) is a cross-sectional view taken along the line B-B of Fig. 8(a). Fig. 9(a) is a perspective view of another antenna apparatus. Fig. 9(b) is a side elevation of the antenna apparatus of Fig. 9(a). Fig. 10(a) is a perspective view of another antenna apparatus. Fig. 10(b) is a cross-sectional view taken along the line A-A of Fig. 10(a). Fig. 10(c) is a side elevation of the antenna apparatus of Fig. 10(a). Fig. 11(a) is a perspective view of another antenna apparatus. Fig. 11(b) is a cross-sectional view taken along the line A-A of Fig. 11(a). Fig. 11(c) is a side elevation of the antenna apparatus of Fig. 11(a). Fig. 12(a) is a perspective view of another antenna apparatus. Fig. 12(b) is a cross-sectional view taken along the line A-A of Fig. 12(a). Fig. 12(c) is a side elevation of the antenna apparatus of Fig. 12(a). Fig. 13(a) is a perspective view of another antenna apparatus. Fig. 13(b) is a cross-sectional view taken along the line A-A of Fig. 13(a). Fig. 13 (c) is a side elevation of the antenna apparatus of Fig. 13(a). The various aspects of the invention are illustrated and explained separately below. Figs. 5(a) to 5(c) schematically illustrate a first antenna arrangement, Fig. 5(a) being a perspective view, Fig. 5(b) a cross-sectional view taken along the line A-A of Fig. 5(a) and Fig. 5(c) a cross-sectional view taken along the line B-B of Fig. 5(a). Although not having all of the features of the invention (described later), it will be described below as it forms the basis for the description of the invention. In these figures, radiation slots 1, 1' are formed respectively on a first set of parallel conductive plates 2, 2' and both conductive plates 2, 2' are connected by a second set of conductive plates 3', 3", 3''' to configurate a rectangular parallelepiped. The inside of the rectangular parallelepiped is filed with a dielectric material 4. The radiation slots 1, 1' are excited by a triplate line 6 formed of the conductive plates 2, 2' and strip lines 5. Numeral 7 designates a coaxial connector for feeding the triplate line; and 8 a coaxial line. The conductive plates 2, 2', 3, 3', 3", 3‴ are grounded. Fig. 6 is a diagram explaining the principle of the antenna apparatus of Fig. 5(a). A signal propagating through the coaxial line 8 enters the triplate line 6 via the coaxial connector 7. The triplate line 6 can be formed in a small size resulting in reduction in size of the antenna apparatus by filling the rectangular parallelepiped with the dielectric material 4. Both ends of the triplate line 6 are connected respectively to the right side edge of the radiation slot 1 and the left side edge of the slot 1' with respect to Fig. 5(b) and a voltage is applied across the strip line 5 and the first set of the ground conductive plates 2, 2'. Since the ends of the triplate line 6 are connected to the opposite side edges of the radiation slots 1, 1', the electric fields inside the rectangular parallelepiped formed of the first set of conductive plates 2, 2' and the second set of conductive plates 3', 3", 3''' are reversed with each other as indicated by the arrow marks in Fig. 6. Therefore, the radiation slots 1, 1' provided on the grounded conductive plates 2, 2' are excited out of phase (in a phase difference of 180 degrees). The radiation field formed by these radiation slots 1, 1' becomes continuous in the horizontal plane (azimuth direction) and a horizontally polarized omnidirectional radiation pattern can be obtained. In this arrangement, the radiation slots 1, l'are fed with the triplate line 6, but another feeding line sch as a coaxial line can also be used for the same purpose. Fig. 7 indicates measured gains of horizontally polarized and vertically polarized waves when the antenna apparatus of Fig. 5(a) is rotated 360 degrees in the horizontal plane. As seen from Fig. 7, in the case of the horizontally polarized wave, an amount of ripple is within 2 dB, resulting in a substantially omnidirectional pattern. The gain of the vertically polarized wave which is a cross-polarized wave is -20 db or less and satisfactory characteristics results. Figs. 8(a) to 8(c) schematically illustrate an antenna apparatus demonstrating one aspect of the present invention, Fig. 8(a) being a perspective view, Fig. 8(b) a cross-sectional view taken along the line A-A and Fig. 8(c) a cross-sectional view taken along the line B-B. In this arrangement a portion 9 of the dielectric material 4 corresponding to the radiation slots 1, 1' is removed. The antenna apparatus of this arrangement also shows, with the same principle as the antenna apparatus of Fig. 5, a horizontally polarized omnidirectional radiation pattern. Since the portion 9 of the dielectric material 4 between the radiation slots 1, 1' formed on the first set of grounded conductive plates 2, 2' is removed, the radiation slots 1, 1' must be longer, in order to have them resonate at the same resonance frequency that those in Fig. 5 wherein no dielectric material 4 is removed, because a wavelength shortening effect by the dielectric material 4 is lost. The radiation slots 1, 1' being set longer, the beam width becomes narrow, the gain in the direction perpendicular to the plates 2, 2' increases and the gain in the horizontal plane can be increased. It is noted that a dielectric material may be provided in a parallelepiped defined by the radiation slots 1, 1'. Figs. 9(a) and 9(b) schematically illustrate a further antenna arrangement, Fig. 9(a) being a perspective view and Fig. 9(b) a side elevation, in which horn-type metal conductors 15, 15' are optionally coupled to upper and lower surfaces of the antenna apparatus. In this arrangement, for the same reason as explained above, a horizontally polarized wave is excited omnidirectionally. If only one radiation slot 1, 1' is formed on each of the conductive plates 2, 2', there is a limitation to a change in beam width in the elevating direction and it is difficult to obtain a high gain. Instead of vertically arranging a plurality of radiation slots on the conductive plates 2, 2' to narrow the beam width in elevation, this antenna employs the horn-type conductors 15, 15' coupled to the upper and lower ends of the antenna apparatus described before. The horn-type conductors 15, 15' operate in combination like a horn antenna. Since the gain of this antenna is determined by a size of the aperture of the horn, a higher gain can be obtained by enlarging the aperture of the horn. This means that a high gain can be obtained even if only one radiation slot is provided on each of the conductive plates 2, 2'. A slant angle α of the horn-type conductors 15, 15' with respect to the horizontal plane does not give any influence on an omnidirectional pattern in the horizontal plane. The beam width and gain in the vertical plane can be easily adjusted by changing the slant angle α. Figs. 10(a) to 10(c) schematically illustrate an antenna demonstrating another aspect of the present invention, Fig. 10(a) being a perspective view, Fig. 10(b) a cross- section taken along the line A-A and Fig. 10(c) a side elevation. In this arrangement a third set of conductive plates 16, 16' is provided that electrically connect the first set of conductive plates 2, 2' of the antenna apparatus. In principle, an omnidirectional radiation pattern can be obtained if a size of the conductive plates 2, 2' is infinite. Since the conductive plates 2, 2' are limited in size, however, a ripple is generated due to the interference of waves diffracted at the edge portions of the conductive plates 2, 2'. The generated ripple changes in the period of about one wavelength depending on the size of the conductive plates 2, 2'. Since the ripple can be minimized by changing the size of the conductor plates 2, 2'. The conductive plates 16, 16' are additionally provided to cover the opposing conductive plates 3, 3" of the antenna apparatus. Whether the spaces between the conductive plates 3, 3" and the third set of conductive plates 16, 16' are filled with a dielectric material or not is optional. Figs. 11(a) to 11(c) schematically illustrate a configuration of a further antenna arrangement, Fig. 11(a) being a perspective view, Fig. 11(b) a cross-section taken along the line A-A and Fig. 11(c) a side elevation. This demonstrates the use of the horn-type conductors 15, 15a as in Figs. 9(a) and 9(b) together with conductive bars 18, 18'. In these figures, the radiation slots 1, 1' are formed to oppose each other on a cylindrical waveguide 17 of which both ends are short-circuited. To one side edge of each of the radiation slots 1, 1' are soldered conductive bars 18, 18'. Numeral 19 designates a waveguide flange. When the circular waveguide 17 is excited in a TM 01 mode, a current flows in the axial direction. If the radiation slots 1, 1' are provided in parallel to the axis of the waveguide 17, the radiation slots 1, 1' are not excited because the slots do not cross the current. The radiation slots 1, 1' can be excited by fixing the conductive bars 18, 18' inside the circular waveguide 17 from the side edges of the radiation slots 1, 1'. A horizontally polarized omnidirectional radiation pattern can be obtained by arranging one or more radiation slots in the circumferential direction of the cylindrical waveguide 17. The beam width in the vertical plane can be narrowed by arranging a plurality of radiation slots in parallel to the longitudinal axis of the circular waveguide 17. Since the radiation slots 1, 1' are excited by exciting the cylindrical conductor 17, a standing wave position deviates when an excitation frequency of the waveguide 17 changes. Then, the amplitude and phase of a signal exciting the radiation slots 1, 1' change and a radiation pattern obtained by combining radiation fields from the slots 1, 1' also changes. It is possible to provide the horn-type conductors 15, 15' to both ends of the circular waveguide 17 in order to obtain a narrower beam width in the vertical plane. In this antenna, the radiation slots 1, 1' are excited using the conductor bars 18, 18', but it is possible to excite the radiation slots 1, 1' by slanting the radiation slots 1, 1' with respect to the axis of the circular waveguide 17. Figs. 12(a) to 12(c) schematically illustrate a configuration of a further antenna arrangement which has the conductive bars 18, 18', Fig. 12(a) being a perspective view, Fig. 12(b) a plan view taken along the line A-A and Fig. 12(c) a side elevation. In this antenna a centre conductor 20 is provided through the circular waveguide 17 to form a coaxial line 17'. If the coaxial line 17' including short-circuited ends is excited in the basic mode (the magnetic field is uniform in the circumferential direction of the coaxial line 17'), a current flows in the longitudinal axial direction. If the radiation slots 1, 1' are provided in parallel to the axis of the coaxial line 17', the radiation slots 1, 1' are not excited. In order that these slots are excited, the conductor bars 18, 18' are provided to protrude inside the coaxial line 17' from the side edges of the radiation slots 1, 1'. A horizontally polarized omnidirectional radiation pattern can be obtained by providing one or more radiation slots in the circumferential direction. In order to make the beam in the vertical direction narrower, a plurality of radiation slots may be arranged in parallel to the axis of the coaxial line 17'. Since the radiation slots 1, 1' are excited by exciting the coaxial line 17' the position of a standing wave is deviated if the excitation frequency of the coaxial line 17' is shifted. Then, the amplitude and phase of a signal for exciting the radiation slots 1, 1' change and a radiation pattern obtained by combining the radiation fields from the slots 1, 1' is also changed. In order to avoid this problem, the horn-type conductors 15, 15' may be provided, as described previously, to both ends of the coaxial line 17' in view of obtaining a narrower beam width in the vertical direction. Figs. 13(a) to 13(c) schematically illustrate a configuration of a further antenna arrangement with the conductive bars 18, 18', Fig. 13(a) being a perspective view, Fig. 13(b) a cross-sectional view taken along the line A-A and Fig. 13(c) a side elevation. In the antenna, the radiation slots 1, 1' are formed on two opposing surfaces of a rectangular waveguide 21. If the rectangular waveguide 21 having short-circuited ends is excited in the TE 01 mode, the radiation slots 1,1' must be formed at positions offset from the longitudinal axis of the waveguide 21 for excitation. Then, a beam tilt is generated like in the prior art and a ripple in the horizontal plane becomes large. In this antenna, the radiation slots 1, 1' are provided in parallel with the centre line of the H plane of the rectangular waveguide 21 and the conductive bars 18, 18' protruding inside the waveguide 21 are fixed to the side edges of the radiation slots 1, 1'. The conductive bars 18, 18' establish a distribution of electromagnetic field asymmetrical with respect to the centre line of the rectangular waveguide 21, whereby the radiation slots 1, 1' provided on the centre line of the plane H are excited, resulting in the generation of an omnidirectional radiation pattern having no beam tilt.

Claims: An antenna apparatus having radiation slots (1, 1') arranged at opposite positions on a grounded conductive rectangular hollow body (2, 2', 3, 3') formed of conductive plates, said hollow body being provided with a dielectric material therein, said radiation slots (1, 1') being excited out of phase to form an omnidirectional radiation pattern in a plane perpendicular to the longitudinal axis of said hollow body, (2, 2', 3, 3'), characterised in that a through-hole (9) is formed extending between said radiation slots (1, 1') and in that semi-cylindrical conductor plates (16, 16') are respectively mounted to the conductive plates (2, 2') parallel to the longitudinal axis of said hollow body for the purpose of reducing any influence of waves diffracted at the edges of the conductive plates (2, 2'). An antenna apparatus according to claim 1, wherein at least one conductive bar is provided around said radiation slots (1, 1') for connecting said opposing conductive plates (2, 2'). An antenna apparatus according to claim 1, wherein horn-type conductor plates (15, 15') are provided to the conductive plates (2, 2') perpendicular to the longitudinal axis of said rectangular hollow body.

Inventors: KATAGI TAKASHI, OHMINE HIROYUKI, SATO SHIN-ICHI, SUNAHARA YONEHIKO, WADAKA SHUSOU, KATAGI, TAKASHI, OHMINE, HIROYUKI, SATO, SHIN-ICHI, SUNAHARA, YONEHIKO, WADAKA, SHUSOU, Katagi, Takashi, Mitsubishi Denki K.K., Ohmine, Hiroyuki, Mitsubishi Denki K.K., Sato, Shin-ichi, Mitsubishi Denki K.K., Sunahara, Yonehiko, Mitsubishi Denki K.K., Wadaka, Shusou, Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1994
Main classification: H01Q13
IPC codes: H01Q21, H01Q13